Add ability to exclude specific fields from fields.Dict during de-/serialization

Some data structures are semi-defined, sitting in-between fields.Dict() and fields.Nested(). It would be helpful to be able to exclude specific fields by name from de-/serialization

[deckar01]

Only knowing the fields you want to exclude seems like an unusual use case. I would not recommend adding this functionality into the core Dict field. The nicer syntax you are looking for could be accomplished by subclassing Dict.

class BlacklistedDict(fields.Dict):
    def __init__(self, blacklist, **kwargs):
        super(BlacklistedDict, self).__init__(**kwargs)
        self.blacklist = blacklist

    def _serialize(self, value, attr, obj):
        value = super(BlacklistedDict, self)._serialize(value, attr, obj)
        return {key: value[key] for key in value if key not in self.blacklist}

    def _deserialize(self, value, attr, obj):
        value = super(BlacklistedDict, self)._deserialize(value, attr, obj)
        return {key: value[key] for key in value if key not in self.blacklist}

[traut]

@lafrech I can but property in fields.Dict constructor would be much prettier, hence this request